U.S. patent application number 14/410169 was filed with the patent office on 2015-11-26 for hybrid self-reinforced composite material.
The applicant listed for this patent is KATHOLIEKE UNIVERSITEIT LEUVEN, UNIVERSITY OF LEEDS. Invention is credited to Mark J. BONNER, Larissa GORBATIKH, Peter J. HINE, Yentl SWOLFS, Ignaas VERPOEST, Ian M. WARD.
Application Number | 20150336333 14/410169 |
Document ID | / |
Family ID | 48699022 |
Filed Date | 2015-11-26 |
United States Patent
Application |
20150336333 |
Kind Code |
A1 |
BONNER; Mark J. ; et
al. |
November 26, 2015 |
HYBRID SELF-REINFORCED COMPOSITE MATERIAL
Abstract
The present invention provides novel hybrid self-reinforced
composites, combining an oriented brittle fibre and an oriented
thermoplastic polymeric ductile fibre (as reinforcement phase) in
the same thermoplastic polymeric matrix phase. The hybrid
self-reinforced composites are strong and stiff, but in case of
impact or crash they have high strain to failure and absorb a lot
of energy. The present invention also relates to methods to produce
said hybrid self-reinforced composites by a hot compaction
technique.
Inventors: |
BONNER; Mark J.; (Leeds,
GB) ; GORBATIKH; Larissa; (Tervuren, BE) ;
HINE; Peter J.; (Leeds, GB) ; SWOLFS; Yentl;
(Minderhout, BE) ; VERPOEST; Ignaas; (Kessel-Lo,
BE) ; WARD; Ian M.; (Leeds, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KATHOLIEKE UNIVERSITEIT LEUVEN
UNIVERSITY OF LEEDS |
Leuven
Leeds |
|
BE
GB |
|
|
Family ID: |
48699022 |
Appl. No.: |
14/410169 |
Filed: |
June 24, 2013 |
PCT Filed: |
June 24, 2013 |
PCT NO: |
PCT/EP2013/063172 |
371 Date: |
December 22, 2014 |
Current U.S.
Class: |
442/202 ;
156/60 |
Current CPC
Class: |
D03D 25/00 20130101;
B29C 70/465 20130101; Y10T 442/3171 20150401; C08J 5/048 20130101;
B29C 70/04 20130101; Y10T 156/10 20150115 |
International
Class: |
B29C 70/04 20060101
B29C070/04; B29C 70/46 20060101 B29C070/46; D03D 25/00 20060101
D03D025/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 22, 2012 |
GB |
1211071.4 |
Jun 22, 2012 |
GB |
1211102.7 |
Jan 11, 2013 |
GB |
1300461.9 |
Jan 14, 2013 |
GB |
1300594.7 |
Claims
1-25. (canceled)
26. A fibrous self-reinforced composite (SRC) material comprising:
(i) a thermoplastic polymer as matrix phase, and a reinforcement
phase comprising (ii) a first oriented polymeric ductile fibre
having the same type as the matrix phase and (iii) a second high
stiffness, brittle fibre, wherein said brittle fibres make up less
than 30 vol % of the composite material and wherein said brittle
fibres are highly dispersed within said composite material by the
ductile fibres and the brittle fibres being organised in an
intralayer configuration, and/or the ductile fibres and the brittle
fibres being organised in an interlayer configuration wherein
alternatingly a layer of ductile fibres and a layer of brittle
fibres is introduced and wherein the thickness of the layer of
brittle fibres is smaller than 125 .mu.m times the square root of
the ratio (230 GPa/stiffness of the brittle fibre), and/or the
ductile fibres and the brittle fibres being organised in an
intrayarn configuration.
27. A fibrous self-reinforced composite (SRC) material according to
claim 26, wherein the ductile fibres and the brittle fibres are
being organised in an intralayer configuration.
28. A fibrous self-reinforced composite (SRC) material according to
claim 27, wherein the intralayer configuration comprises within at
least one layer, at least a plurality of brittle fibres are
introduced in substantially parallel bands, the substantially
parallel bands being spaced from each other.
29. A fibrous self-reinforced composite (SRC) material according to
claim 28, wherein the spacing between parallel bands in the layer
is larger than the average width of the parallel bands.
30. A fibrous self-reinforced composite (SRC) material according to
claim 28, wherein the spacing between parallel bands in the layer
is at least 5 mm.
31. A fibrous self-reinforced composite (SRC) material according to
claim 28, wherein the brittle fibres are present in a first set of
parallel bands according to a first orientation and wherein the
ductile fibres are present in a second set of parallel bands
according to a second orientation, perpendicular to the first
orientation.
32. A fibrous self-reinforced composite (SRC) material according to
claim 28, wherein within at least one layer the brittle fibres are
configured in woven bands.
33. A fibrous self-reinforced composite (SRC) material according to
claim 28, wherein in at least one layer at least one the brittle
fibres are configured in a third set of parallel bands and the
ductile fibres are present in a fourth set of parallel bands, the
third set of parallel bands and the fourth set of parallel bands
having the same orientation.
34. A fibrous self-reinforced composite (SRC) material according to
claim 26, wherein the ductile fibres and the brittle fibres are
being organised in an interlayer configuration wherein
alternatingly a layer of ductile fibres and a layer of brittle
fibres is introduced and wherein the thickness of the layer of
brittle fibres is smaller than 125 .mu.m times the square root of
the ratio (230 GPa/stiffness of the brittle fibre).
35. A fibrous self-reinforced composite (SRC) material according to
claim 34, wherein the thickness of the layers of brittle fibres is
less than 125 .mu.m.
36. A fibrous self-reinforced composite (SRC) material according to
claim 26, wherein the ductile fibres and the brittle fibres being
organised in an intrayarn configuration.
37. A fibrous self-reinforced composite (SRC) material according to
claim 26, wherein the brittle fibres are highly dispersed such that
the composite material has at least twice the stiffness,
substantially a same or higher strength and at least 0.8 times the
failure strain of a self-reinforced composite reference material
having no brittle fibres but further having the same composition as
the fibrous self-reinforced composite material.
38. A fibrous self-reinforced composite (SRC) material according to
claim 26, wherein said ductile fibre has a failure strain of at
least 8% and wherein said brittle fibre has a failure strain of
less than 4%.
39. A fibrous self-reinforced composite (SRC) material according to
claim 26, wherein said composite material has a stiffness of at
least 10 GPa, a tensile strength of at least 250 MPa, and an impact
(Izod) strength of at least 2500 J/m.
40. A fibrous self-reinforced composite (SRC) material according to
claim 26, wherein the matrix phase is produced by hot
compaction.
41. A fibrous self-reinforced composite (SRC) material according to
claim 40, wherein the matrix phase is produced by selectively
melting a fraction of the surface of each oriented polymer
element.
42. A fibrous self-reinforced composite (SRC) material according to
claim 26, wherein said ductile fibre is a thermoplastic polyolefin,
such as PP or PE, or a thermoplastic polyester, or a thermoplastic
polyamide.
43. A fibrous self-reinforced composite (SRC) material according to
claim 26, wherein said brittle fibre is any of a carbon, glass or
natural fibre including a flax fibre.
44. A fibrous self-reinforced composite (SRC) material according to
claim 26, wherein said brittle fibres are uniformly distributed
within said composite material.
45. A method for manufacturing a fibrous self-reinforced composite
material, the method comprising providing a thermoplastic polymer
as matrix phase, and a reinforcement phase comprising a first
oriented polymeric ductile fibre having the same type as the matrix
phase and a second high stiffness, brittle fibre, wherein said
brittle fibres make up less than 30 vol % of the composite
material, wherein said providing comprises providing said brittle
fibres in a highly dispersed within said composite material by
organising the ductile fibres and the brittle fibres in an
intralayer configuration, or organising the ductile fibres and the
brittle fibres in an interlayer configuration wherein alternatingly
a layer of ductile fibres and a layer of brittle fibres is
introduced and wherein the thickness of the layer of brittle fibres
is smaller than 125.mu..pi..times.square root of (230 GPa/stiffness
of the brittle fibre), or organising the ductile fibres and the
brittle fibres in an intrayarn configuration.
46. A method according to claim 45, wherein said providing
comprises the steps of (a) making an assembly of brittle and
polymeric ductile fibres and (b) subjecting said assembly to a hot
compaction step.
47. A method according to claim 45, the method comprising
maintaining the temperature of the assembly during said hot
compaction step within a melting range of the polymer fibres as
measured by differential scanning calorimetry.
48. A product comprising a fibrous self-reinforced composite (SRC)
material according claim 26.
49. A fibrous self-reinforced composite material manufactured using
a method according to claim 45.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to hybrid self-reinforced
composite materials, and methods of making and using said composite
materials, as well as to products comprising such hybrid
self-reinforced composite materials.
BACKGROUND OF THE INVENTION
[0002] A composite is a combination of a fibre and a matrix. The
ideal objective for any composite designer is to have available a
material that combines high stiffness/strength with high
toughness/extensibility. In general, however, these two types of
behaviour are mutually exclusive. Traditional carbon fibre
composites deliver exceptional stiffness and strength but can often
have a limited extensibility and poor damage tolerance especially
at lower operating temperatures (vehicles need to maintain their
integrity and crash performance at -40.degree. C.) (with failure
strains limited to a few percent, in line with the failure strain
of the carbon fibre). On the other hand, the development of
self-reinforced composites over the last 20 years has seen the
emergence of a new material that is light weight and has
exceptional toughness even at low temperatures.
[0003] Self-reinforced composites, also referred to as
self-reinforced composites (or SRCs in short) are composites where
the fibre and matrix are made out of the same polymer. Capiati
& Porter [Capiati, N. J.; Porter, R. S., Journal of Materials
Science, 1975, 10, 1671] combined drawn polyethylene fibres with a
polyethylene matrix with a lower melting point. This allowed them
to impregnate the polyethylene fibres without melting them. While
there are a number of published methods for producing
self-reinforced polymer composites (for example film stacking,
powder impregnation and bicomponent tapes), another alternative
process for producing such composites is the hot compaction process
[Ward, I. M.; Hine, P. J., Polymer, 2004, 45, 1413; GB2253420]. The
underlying principle is to take assemblies of oriented
single-component polymer fibres or tapes, and expose them to the
right temperature, pressure and time conditions, such that a thin
skin on the surface of each oriented element is `selectively
melted`. On subsequent fast cooling, the melted material
recrystallises to form the matrix phase of a self-reinforced
polymer composite, with the remaining fraction of the original
oriented phase acting as the reinforcement. The virtues of this
technique are that the matrix phase is produced around each fibre,
negating the need for infiltration. This avoids impregnation
problems, as the matrix is created in situ. In addition, molecular
continuity is achieved between the two components of the final
composite, which gives a very strong bond between the two phases.
Research has shown this to work with a wide range of oriented
thermoplastic fibres and tapes including polyethylene,
polypropylene, polyester and nylon.
[0004] Self-reinforced polypropylene (SRPP), particularly when made
by the hot compaction process, has the biggest potential, mainly
due to its low price, low density, high toughness and broad
processing window. Heavily drawn polypropylene (PP) tapes are used,
with a stiffness of about 10 GPa, a strength of 500-600 MPa and a
failure strain of 10-15% (see e.g. [Ward, I. M.; Hine, P. J.,
Polymer, 2004, 45, 1413]). The combination of the high stiffness
and strength of the fibres, the ductility of the recrystallized
matrix and the perfect fibre-matrix bonding results in a very high
toughness, both in terms of failure strain (20%) and impact
resistance (notched Izod impact strength of 4750 J/m) (see e.g.
www.curvonline.com).
[0005] However, for more widespread application in structural
parts, SRPP has one major disadvantage: the stiffness of the
compacted weaves is low (3-5 GPa, see [Ward, I. M.; Hine, P. J.,
Polymer, 2004, 45, 1413]) compared to glass and carbon fibre
composites (20-80 GPa).
[0006] Accordingly, in general, self-reinforced composites are very
tough materials, but they lack stiffness. Classic fibre-reinforced
composites on the other hand are very stiff materials, but they
lack toughness. Most polymer composites are either tough but
compliant or stiff but brittle.
[0007] When a second type of fibre is added to the composite, this
is called hybridizing. Many reports deal with the properties and
production of a three component hybrid composite, i.e. a brittle
fibre, a ductile fibre and a third matrix phase (of a different
material than the fibres). The resulting properties of the hybrid
composite are not easy to deduce. Sometimes they follow the rule of
mixtures (i.e. the properties of the hybrid composite material can
be estimated based on the assumption that a composite property is
the volume weighed average of the properties of the components).
However, deviations from the rule of mixtures have been reported as
well. In this respect, a positive or negative deviation of a
property from the rule of mixture is defined as a positive or
negative hybrid effect. The term hybrid effect has been used to
describe the phenomenon of an apparent (positive or negative)
synergistic effect in the properties of a composite containing two
or more types of fibre.
[0008] Few reports deal with the hybridization of self-reinforced
composites. These studies generally have looked at combining
discrete layers of a prepreg composite and a self-reinforced
polymer (SRP) sheet, in casu self-reinforced polypropylene (SRPP).
Taketa et al. [Taketa et al, Composites Part A: Applied Science and
Manufacturing, 2010, 41, 927] used a pre-impregnated carbon fibre
weave, with an areal density of the fabric is 285 g/m.sup.2. Fabich
et al. [Fabich et al (2010). Toughness Improvement in Hybrid
Composites Made of Carbon Fibre Reinforced Polypropylene and
Self-Reinforced Polypropylene. Lancaster: Destech Publications,
Inc.] used a unidirectional carbon fibre prepreg of 0.25 mm. The
research of Kuan et al. [Kuan et al. Malaysian Polymer Journal,
2009, 4, 71] used unidirectional glass fibre polypropylene (GFPP)
prepreg, but does not specify any other details of the prepreg.
Only Kuan et al. show tensile curves of the hybrid SRPP. The
failure strain of the glass fibre layers are clearly lower in the
hybrids, demonstrating that Kuan et al. found a negative hybrid
effect for failure strain.
[0009] The above shows that there is still a need in the art to
have available a material that combines high stiffness/strength
with high toughness/extensibility.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to provide good
hybrid self-reinforced composite materials, as well as to provide
good methods for manufacturing these and products comprising such
hybrid self-reinforced composite materials.
[0011] It is an advantage of embodiments of the present invention
that composite materials are provided that substantially maintain
the ductile character of the composite material, even if failure of
the brittle fibres occurs. Such an advantage is obtained by
embodiments of the present invention by using a material comprising
ductile fibres and brittle fibres configured in one of three
configurations (i.e. an interlayer configuration, an intralayer
configuration or an intrayarn configuration) providing alternative
solutions for maintaining the ductile character of the composite
material even if failure of the brittle fibres occurs.
[0012] The above objective is accomplished by a method and device
according to the present invention.
[0013] The present invention relates to a fibrous self-reinforced
composite (SRC) material comprising
(i) a thermoplastic polymer as matrix phase, and a reinforcement
phase comprising (ii) a first oriented polymeric ductile fibre
having the same type as the matrix phase and (iii) a second high
stiffness, brittle fibre, wherein said brittle fibres make up less
than 30 vol % of the composite material and wherein said brittle
fibres are highly dispersed within said composite material by
[0014] the ductile fibres and the brittle fibres being organised in
an intralayer configuration, and/or
[0015] the ductile fibres and the brittle fibres being organised in
an interlayer configuration wherein alternatingly a layer of
ductile fibres and a layer of brittle fibres is introduced and
wherein the thickness of the layer of brittle fibres is smaller
than 125 .mu.m times the square root of (230 GPa/stiffness of the
brittle fibre), and/or
[0016] the ductile fibres and the brittle fibres being organised in
an intrayarn configuration.
The ductile fibres and the brittle fibres may be organised in an
intralayer configuration. The intralayer configuration may comprise
within at least one layer, at least a plurality of brittle fibres
are introduced in substantially parallel bands, the substantially
parallel bands being spaced from each other.
[0017] The spacing between parallel bands in the layer may be
larger than the average width of the parallel bands. The spacing
between parallel bands may be larger than twice the average width
of the parallel bands.
[0018] The spacing between parallel bands in the layer may be at
least 5 mm.
[0019] The brittle fibres may be present in a first set of parallel
bands according to a first orientation and the ductile fibres may
be present in a second set of parallel bands according to a second
orientation, perpendicular to the first orientation.
[0020] Within at least one layer the brittle fibres may be
configured in woven bands.
[0021] In at least one layer at least one the brittle fibres may be
configured in a third set of parallel bands and the ductile fibres
are present in a fourth set of parallel bands, the third set of
parallel bands and the fourth set of parallel bands having the same
orientation.
[0022] The ductile fibres and the brittle fibres may be organised
in an interlayer configuration wherein alternatingly a layer of
ductile fibres and a layer of brittle fibres is introduced and
wherein the thickness of the layer of brittle fibres is smaller
than 125 .mu.m times the square root of the ratio (230
GPa/stiffness of the brittle fibre).
[0023] The thickness of the layers of brittle fibres may be less
than 125 .mu.m.
[0024] The ductile fibres and the brittle fibres may be organised
in an intrayarn configuration.
[0025] The brittle fibres may be highly dispersed such that the
composite material has at least twice the stiffness, substantially
a same or higher strength and at least 0.8 times the failure strain
of a self-reinforced composite reference material having no brittle
fibres but further having the same composition as the fibrous
self-reinforced composite material.
[0026] Said ductile fibre may have a failure strain of at least 8%
and wherein said brittle fibre has a failure strain of less than
4%.
[0027] Said composite material may have a stiffness of at least 10
GPa, a tensile strength of at least 100 MPa, e.g. at least 120 MPa,
in some embodiments at least 250 MPa, and an impact (Izod) strength
of at least 2500 J/m.
[0028] The matrix phase may be produced by hot compaction.
[0029] The matrix phase may be produced by selectively melting a
fraction of the surface of each oriented polymer element.
[0030] Said ductile fibre may be a thermoplastic polyolefin, such
as PP or PE, or a thermoplastic polyester, or a thermoplastic
polyamide.
[0031] Said brittle fibre may be any of a carbon, glass or natural
fibre such as for example flax fibre.
[0032] Said brittle fibres may be uniformly distributed within said
composite material.
[0033] The present invention also relates to a method for
manufacturing a fibrous self-reinforced composite material, the
method comprising providing a thermoplastic polymer as matrix
phase, and a reinforcement phase comprising a first oriented
polymeric ductile fibre having the same type as the matrix phase
and a second high stiffness, brittle fibre, wherein said brittle
fibres make up less than 30 vol % of the composite material,
wherein said providing comprises providing said brittle fibres in a
highly dispersed within said composite material by
[0034] organising the ductile fibres and the brittle fibres in an
intralayer configuration, or
[0035] organising the ductile fibres and the brittle fibres in an
interlayer configuration wherein alternatingly a layer of ductile
fibres and a layer of brittle fibres is introduced and wherein the
thickness of the layer of brittle fibres is smaller than 125
.mu.m.times.square root of (230 Gpa/stiffness of the brittle
fibre), or
[0036] organising the ductile fibres and the brittle fibres in an
intrayarn configuration.
[0037] Providing may comprise the steps of (a) making an assembly
of brittle and polymeric ductile fibres and (b) subjecting said
assembly to a hot compaction step.
[0038] The method may comprise maintaining the temperature of the
assembly during said hot compaction step within a melting range of
the polymer fibres as measured by differential scanning
calorimetry.
[0039] The present invention also relates to a product comprising a
fibrous self-reinforced composite (SRC) material as described
above.
[0040] The present invention also relates to a fibrous
self-reinforced composite material manufactured using a method as
described above.
[0041] In one aspect, the present invention also relates to a
fibrous self-reinforced composite (SRC) material comprising
(i) a thermoplastic polymer as matrix phase, and a reinforcement
phase comprising (ii) a first oriented polymeric ductile fibre
having the same type as the matrix phase and (iii) a second high
stiffness, brittle fibre, wherein said brittle fibres make up less
than 30 vol % of the composite material and wherein said brittle
fibres are highly dispersed within said composite material such
that the composite material has at least twice the stiffness,
substantially a same or higher strength and at least 0.8 times the
failure strain of a self-reinforced composite reference material
having no brittle fibres but further having the same composition as
the fibrous self-reinforced composite material.
[0042] The present invention also relates to a fibrous
self-reinforced composite (SRC) material comprising
(i) a thermoplastic polymer as matrix phase, and a reinforcement
phase comprising (ii) a first oriented polymeric ductile fibre
having the same type as the matrix phase and (iii) a second high
stiffness, brittle fibre, wherein said brittle fibres make up less
than 30 vol % of the composite material and wherein said brittle
fibres are highly dispersed within said composite material by
organising the ductile fibres and the brittle fibres in an
intralayer configuration.
[0043] The present invention furthermore relates to a fibrous
self-reinforced composite (SRC) material comprising
(i) a thermoplastic polymer as matrix phase, and a reinforcement
phase comprising (ii) a first oriented polymeric ductile fibre
having the same type as the matrix phase and (iii) a second high
stiffness, brittle fibre, wherein said brittle fibres make up less
than 30 vol % of the composite material and wherein said brittle
fibres are highly dispersed within said composite material by
organising the ductile fibres and the brittle fibres in an
interlayer configuration wherein alternatingly a layer of ductile
fibres and a layer of brittle fibres is introduced and wherein the
thickness of the layer of brittle fibres is smaller than 125
.mu.m.times.the square root of the ratio (230 Gpa/stiffness of the
brittle fibre).
[0044] The present invention also relates to a fibrous
self-reinforced composite (SRC) material comprising
(i) a thermoplastic polymer as matrix phase, and a reinforcement
phase comprising (ii) a first oriented polymeric ductile fibre
having the same type as the matrix phase and (iii) a second high
stiffness, brittle fibre, wherein said brittle fibres make up less
than 30 vol % of the composite material and wherein said brittle
fibres are highly dispersed within said composite material by
organising the ductile fibres and the brittle fibres in an
intrayarn configuration.
[0045] The present invention furthermore relates to a process for
manufacturing a fibrous self-reinforced composite material
comprising the steps of (i) making an assembly of brittle and
polymeric ductile fibres and (ii) subjecting said assembly to a hot
compaction step.
[0046] The temperature at which the assembly is maintained may be
within the melting range of the polymer fibres as measured by
differential scanning calorimetry.
[0047] More in particular, the temperature at which the assembly is
maintained is a temperature between the onset of the melting
endotherm and the end of the melting endotherm of a constrained
oriented polymer fibre or tape as measured by DSC.
[0048] The present invention also relates to a hybrid
self-reinforced composite materials having improved tensile and/or
flexural properties comprising (i) a thermoplastic polymer as
matrix phase, and a reinforcement phase comprising (ii) an oriented
polymeric ductile fibre having the same type as the matrix phase
and having a failure strain of at least 8%; and (iii) a(n)
(oriented) high stiffness, brittle fibre having a failure strain of
less than 4%, wherein said brittle fibres make up less than 30 vol
% of the composite material and wherein said brittle fibres are
highly dispersed within said composite material.
[0049] The fibrous self-reinforced composite material according to
the present invention has a stiffness of at least 6 Gpa such as for
example at least 10 Gpa, a tensile strength of at least 100 MPa,
e.g. at least 120 MPa or at least 250 MPa, and an impact (Izod) of
at least 2500 J/m.
[0050] In one embodiment the brittle and ductile fibres in said
fibrous self-reinforced composite material are organised in an
interlayer configuration wherein the thickness of the layers is
less than 125 .mu.m times the square root of the ratio (230
GPa/stiffness of the brittle fibre). In some embodiments the
thickness of the layer may be smaller than 200 .mu.m, preferably
less than 150 .mu.m or less than 100 .mu.m.
[0051] In another embodiment the brittle and ductile fibres in said
fibrous self-reinforced composite material are organised in an
intralayer configuration.
[0052] In yet another embodiment the brittle and ductile fibres in
said fibrous self-reinforced composite material are organised in an
intrayarn configuration. Said commingled yarns may in its turn be
present in a fabric, film or mat.
[0053] Preferably, said ductile fibre is made up of a thermoplastic
polyolefin, such as PP or PE, a thermoplastic polyester, or a
thermoplastic polyamide. Preferably, said brittle fibre is a
carbon, glass or a natural fibre such as for example flax
fibre.
[0054] Particular and preferred aspects of the invention are set
out in the accompanying independent and dependent claims. Features
from the dependent claims may be combined with features of the
independent claims and with features of other dependent claims as
appropriate and not merely as explicitly set out in the claims.
[0055] These and other aspects of the invention will be apparent
from and elucidated with reference to the embodiment(s) described
hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] FIG. 1a to FIG. 1d schematically represents different
hybridization configurations, wherein FIG. 1a (i) and FIG. 1b
illustrates an interlayer or layer-by-layer or interplay
configuration, FIG. 1a (ii) and FIG. 1c illustrates an intralayer
or cowoven configuration, and FIG. 1a (iii) and FIG. 1d illustrates
an intrayarn or commingled configuration, according to an
embodiment of the present invention.
[0057] FIG. 2 shows the tensile tests on PA6 based intra-layer
hybrids with varying carbon fibre volume fraction, illustrating
features and advantages of embodiments of the present
invention.
[0058] FIG. 3 shows the effect of the width of the reinforcing
prepreg tapes on the hybrid carbon fibre SRPA performance,
illustrating features and advantages of embodiments of the present
invention.
[0059] FIG. 4 compares the tensile properties of a co-woven sample
and a simple interlayer sample (4 vol % carbon fibre for this
comparison), illustrating advantages of different configurations
according to embodiments of the present invention.
[0060] FIG. 5 compares the tensile and bending properties of a
hybrid SRPA/carbon fibre sample, illustrating features and
advantages of embodiments of the present invention.
[0061] FIG. 6a and FIG. 6b shows the tensile properties (FIG. 6a)
and bending properties (FIG. 6b) of a pure PA6 sheet and a hybrid
sample, thereby illustrating features and advantages of embodiments
of the present invention.
[0062] FIG. 7 shows the tensile behaviour of multidirectional
interlayer SRPP/CFPP hybrids, thereby illustrating features and
advantages of embodiments of the present invention.
[0063] FIG. 8 shows the influence of interleaved films on the
tensile behaviour of interlayer CFPP/SRPP hybrids, thus
illustrating features and advantages of embodiments of the present
invention.
[0064] FIG. 9 illustrates a comparison of the tensile behaviour of
intralayer SRPA6 and intralayer SRPP hybrids (CF content of ca. 4
vol %), thus illustrating features and advantages of embodiments of
the present invention.
[0065] FIG. 10 represents the tensile behaviour of intralayer
CFPP/SRPP hybrids, thus illustrating features and advantages of
embodiments of the present invention.
[0066] FIG. 11 shows the tensile behaviour of intralayer CFPP/SRPP
hybrids with CF in both directions, illustrating features and
advantages of embodiments of the present invention.
[0067] FIG. 12 shows the tensile behaviour of CF/PA12 intrayarn
hybrids, illustrating features and advantages of embodiments of the
present invention.
[0068] FIG. 13 shows the tensile properties of braided CF/PA12
intrayarn cloth, thus illustrating features and advantages of
embodiments of the present invention.
[0069] FIG. 14 compares the tensile properties for different weave
styles for the intrayarn samples, thus illustrating features and
advantages of embodiments of the present invention.
[0070] FIG. 15 illustrates the influence on the tensile diagram of
the degree of dispersion of brittle fibres in a particular setup,
illustrating an advantage of embodiments of the present
invention.
[0071] FIG. 16 illustrates the influence of carbon fibre volume
fraction on the tensile diagram of intralayer SRPP/CFPP hybrids,
illustrating features and advantages of embodiments of the present
invention.
[0072] FIG. 17 illustrates the failure strain of the hybrid cloths,
compared to the non-hybrid reference cloth, illustrating features
and advantages of embodiments of the present invention.
[0073] FIG. 18 illustrates the penetration impact resistance of the
hybrid cloths, compared to non-hybrid reference cloths,
illustrating features and advantages of embodiments of the present
invention.
[0074] FIG. 19 illustrates a schematic tensile diagram with
illustration of the relevant tensile properties, illustrating
features of embodiments of the present invention.
[0075] FIG. 20 illustrates tensile diagrams of the reference
materials, as can be used for illustrating features of embodiments
of the present invention.
[0076] FIG. 21 illustrates tensile diagrams of the 0.degree. and
0.degree./90.degree. plain weave intralayer hybrids illustrating
features of embodiments of the present invention.
[0077] FIG. 22 illustrates a comparison between prediction and
measurements of 0.degree. plain weave intralayer hybrids, thus
illustrating features and advantages of embodiments of the present
invention.
[0078] FIG. 23 shows tensile diagrams of 0.degree./90.degree.
intralayer hybrids in plain and satin weave, illustrating features
of embodiments of the present invention.
[0079] FIG. 24 shows tensile diagrams of 0.degree./90.degree. plain
weave intralayer hybrids with and without film.
[0080] The drawings are only schematic and are non-limiting. In the
drawings, the size of some of the elements may be exaggerated and
not drawn on scale for illustrative purposes.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0081] The present invention will be described with respect to
particular embodiments and with reference to certain drawings but
the invention is not limited thereto but only by the claims. The
drawings described are only schematic and are non-limiting. In the
drawings, the size of some of the elements may be exaggerated and
not drawn on scale for illustrative purposes. The dimensions and
the relative dimensions do not correspond to actual reductions to
practice of the invention.
[0082] Furthermore, the terms first, second and the like in the
description and in the claims, are used for distinguishing between
similar elements and not necessarily for describing a sequence,
either temporally, spatially, in ranking or in any other manner. It
is to be understood that the terms so used are interchangeable
under appropriate circumstances and that the embodiments of the
invention described herein are capable of operation in other
sequences than described or illustrated herein.
[0083] Moreover, the terms top, under and the like in the
description and the claims are used for descriptive purposes and
not necessarily for describing relative positions. It is to be
understood that the terms so used are interchangeable under
appropriate circumstances and that the embodiments of the invention
described herein are capable of operation in other orientations
than described or illustrated herein.
[0084] It is to be noticed that the term "comprising", used in the
claims, should not be interpreted as being restricted to the means
listed thereafter; it does not exclude other elements or steps. It
is thus to be interpreted as specifying the presence of the stated
features, integers, steps or components as referred to, but does
not preclude the presence or addition of one or more other
features, integers, steps or components, or groups thereof. Thus,
the scope of the expression "a device comprising means A and B"
should not be limited to devices consisting only of components A
and B. It means that with respect to the present invention, the
only relevant components of the device are A and B.
[0085] Reference throughout this specification to "one embodiment"
or "an embodiment" means that a particular feature, structure or
characteristic described in connection with the embodiment is
included in at least one embodiment of the present invention. Thus,
appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily all referring to the same embodiment, but may.
Furthermore, the particular features, structures or characteristics
may be combined in any suitable manner, as would be apparent to one
of ordinary skill in the art from this disclosure, in one or more
embodiments.
[0086] Similarly it should be appreciated that in the description
of exemplary embodiments of the invention, various features of the
invention are sometimes grouped together in a single embodiment,
figure, or description thereof for the purpose of streamlining the
disclosure and aiding in the understanding of one or more of the
various inventive aspects. This method of disclosure, however, is
not to be interpreted as reflecting an intention that the claimed
invention requires more features than are expressly recited in each
claim. Rather, as the following claims reflect, inventive aspects
lie in less than all features of a single foregoing disclosed
embodiment. Thus, the claims following the detailed description are
hereby expressly incorporated into this detailed description, with
each claim standing on its own as a separate embodiment of this
invention.
[0087] Furthermore, while some embodiments described herein include
some but not other features included in other embodiments,
combinations of features of different embodiments are meant to be
within the scope of the invention, and form different embodiments,
as would be understood by those in the art. For example, in the
following claims, any of the claimed embodiments can be used in any
combination.
[0088] In the description provided herein, numerous specific
details are set forth. However, it is understood that embodiments
of the invention may be practiced without these specific details.
In other instances, well-known methods, structures and techniques
have not been shown in detail in order not to obscure an
understanding of this description.
[0089] Where in embodiments of the present invention reference is
made to dispersion reference is not made to chemical dispersion,
but to the spread of material in the configuration as discussed for
the different configurations considered below.
[0090] Where in embodiments of the present invention reference is
made to an interlayer configuration, reference is made to a
configuration wherein layers with a different composition are
stacked. Such layers may be arranged alternatingly. Such a
configuration also may be referred to as layer-by-layer
configuration or interplay configuration. By way of illustration,
an example thereof is shown in FIG. 1a (i) and in FIG. 1b.
According to at least some embodiments of the present invention
such a configuration typically comprises alternatingly a layer
comprising ductile fibres and a layer comprising brittle fibres.
Where reference is made to a highly dispersed configuration for the
brittle fibres in this configuration, typically reference is made
to a situation wherein the layers comprising brittle fibres are
dispersed in the layers comprising ductile fibres. Advantageously,
for having a high degree of dispersion, the thickness of the layer
comprising brittle fibres is small. The inverse of the thickness of
the layer comprising brittle fibres, or the average thereof if it
varies over different layers, may be used as a measure for the
degree wherein the configuration is considered dispersed. With
reference to FIG. 1b, a measure for the degree of dispersion thus
may be given by 1/d2, with d2 being the thickness of the layer
comprising brittle fibres.
[0091] Where in embodiments of the present invention reference is
made to an intralayer configuration, reference is made to a
configuration wherein within a same layer, portions such as for
example bands, with different composition may occur. By way of
illustration, an example thereof is shown in FIG. 1a (ii) and FIG.
1c. Where in this configuration reference is made to highly
dispersed configuration reference is made to the degree of
dispersion of bands within a layer. Typically, such a layer may
exist of portions comprising and the degree of dispersion for the
brittle fibres may be expressed by the inverse of the distance
between the portions in the layer comprising brittle fibres. In
some embodiments, in an intralayer configuration, different
portions such as e.g. bands may be woven with respect to each
other. In some embodiments having an intralayer configuration,
portions, e.g. bands, comprising fibres of a first type may be
arranged so that they have a first orientation and portions, e.g.
bands, comprising fibres of a second type may be arranged so that
they have a second orientation substantially parallel.
[0092] Where in embodiments of the present invention reference is
made to an intrayarn configuration, reference is made to a
configuration wherein a yarn is a bundle of fibres whereby brittle
and ductile ductile fibres are intermingled within the yarn. Using
traditional composite performing techniques result in fibres being
present in different directions. An example of such a configuration
is shown in FIG. 1a(iii) and in FIG. 1d. A measure for the degree
of dispersion may be the average distance between a fibre and its
shortest neighbour.
[0093] The above configurations illustrate the basic configurations
for the alternative solutions that could be used according to
embodiments of the present invention. It nevertheless is to be
noticed that also a combination of different solutions is within
the scope of the present invention.
[0094] Where in embodiments or the claims of the present invention
reference is made to a self-reinforced composite, reference may
equally be made to self-reinforced polymer, single polymer
composite or all polymer composite.
[0095] Where in embodiments of the present invention reference is
made to a polymer fibre having the same type as the matrix phase,
reference may be made to a polymer that has the same composition or
to a polymer of the same type but not necessarily of the same
grade, such as e.g. a copolymer.
[0096] Where in embodiments of the present invention reference is
made to a polymer element, reference is made to a polymer fibre or
polymer tape.
[0097] Unidirectional or uniaxial composites have all fibres
oriented in the same direction. They can be produced by an
interlayer or intrayarn strategy. In contrast, in multidirectional
composites the fibres are oriented in at least two directions (e.g.
0.degree. and 90.degree., but other orientations are known in the
art as well). Multidirectional composites can be produced in three
ways: (a) `UD cross ply` in which all the fibres of one layer are
in the same direction, while the fibres of a different layer are in
a different direction (e.g. the unidirectional layers lie in
various 0.degree. and 90.degree.); (b) using woven material; (c)
using an `intralayer` strategy.
[0098] Where in embodiments of the present invention reference is
made to a high stiffness, brittle fibre, reference is made to a
brittle fibre having a longitudinal tensile modulus of 30 GPa or
higher.
[0099] In a first aspect, the present invention is based on the
finding that selected hybrid self-reinforced composite materials
comprising (i) a thermoplastic polymer matrix, (ii) (oriented)
polymeric ductile fibres of the same material as the matrix; and
(iii) (oriented) high stiffness, brittle fibres, can provide
improved properties over the prior art. According to some
embodiments of the present invention, even at low concentrations of
such brittle fibres in the hybrid self-reinforced composite, e.g.
at a concentration less than 30 vol % of the composite material,
these brittle fibres can contribute significantly to improved
tensile and/or flexural properties and/or impact strength of the
hybrid composite materials when the brittle fibres are well
dispersed within the composite. Advantageously, the brittle fibres
are well mixed with the ductile fibres and/or are provided in such
a manner that there is good adhesion between the brittle fibres and
the matrix material.
[0100] Therefore, embodiments of the present invention relate to a
fibrous self-reinforced composite (SRC) material comprising a
thermoplastic polymer as matrix phase, and a reinforcement phase.
The reinforcement phase comprises a first oriented polymeric
ductile fibre having the same type as the matrix phase and a second
high stiffness, brittle fibre, wherein said brittle fibres make up
less than 30 vol % of the composite material. Furthermore,
according to embodiments of the present invention, the brittle
fibres are highly dispersed within said composite material. The
latter is obtained by providing the ductile fibres and the brittle
fibres in an intralayer configuration, and/or in an interlayer
configuration wherein alternatingly a layer of ductile fibres and a
layer of brittle fibres is introduced and wherein the thickness of
the layer of brittle fibres is smaller than 125 .mu.m times the
square root of the ratio (230 GPa/stiffness of the brittle fibre),
and/or in an intrayarn configuration.
[0101] Without being bound by theory, when brittle and ductile
fibres are well mixed, the breaking or damaging of the brittle
fibre has less impact on the properties of the composite material,
and the residual strength and load bearing capacity of the
composite after failure of the brittle fibre is increased. A
similar effect also may be obtained by inducing a good adhesion
between the brittle fibres and the matrix, e.g. by making the
composite through hot compaction.
[0102] The combination of an oriented brittle and an oriented
thermoplastic ductile fibre in hybrid self-reinforced composites
allows for a dual behaviour. In regular use, the hybrid
self-reinforced composite is both strong and stiff. In case of
impact or crash, the hybrid self-reinforced composite still behaves
ductile and absorbs a lot of energy. This dual behaviour is
especially interesting in structural applications, where impact
absorption is important. This makes the automotive industry a key
customer for hybrid SRCs. Also, these hybrid SRCs can be used in
high volume applications, like again the automotive industry.
Because hybrid SRCs are thermoplastic instead of thermoset, they do
not require a curing time. Moreover, because the matrix can be
created in situ, the classic problem of thermoplastic composites,
namely the impregnation, can be avoided.
[0103] According to embodiments of the present invention, the terms
"polymeric ductile fibre" or "ductile fibre" is used herein in a
broad sense to denote strands comprising a thermoplastic polymer,
however formed, with a failure strain of at least 8%, e.g. at least
9% or e.g. at least 10%. The ductile fibres may be non-woven fibres
laid in a web, or may be comprised within yarns, or constituted by
bands or fibrillated tapes, for example formed by slitting films,
depending on the configuration. If comprised within yarns or
constituted by bands or fibrillated tapes, those yarns, bands or
fibrillated tapes may be laid together or they may be formed into a
fabric, for example by weaving or knitting. Suitably, the fibres
used in the process of the invention are formed from molten
polymer, for example as melt spun filaments. However, the oriented
polymer fibres may be obtained by any of the known manufacturing
processes (e.g. melt spinning and drawing and gel spinning and
drawing). Typically, such fibres may have a diameter in the range
0.005 to 0.05 mm. In one embodiment, said yarns, bands, fibrillated
tapes or fabric comprise said brittle fibre as well, e.g. in the
intrayarn configuration.
[0104] In the context of the present invention the polymeric
material that make up the ductile fibre and the matrix phase in the
hybrid composites, is a thermoplastic polymer, such as a
thermoplastic polyolefin. The polyolefin polymer includes
polyethylene, polypropylene or polybutylene, or copolymers
comprising at least one of those olefin polymers. The polyolefin
polymer may be a polypropylene homopolymer or a copolymer
containing a major proportion of polypropylene. Advantageously, it
may be a polyethylene homopolymer or a copolymer containing a major
proportion of polyethylene.
[0105] Other classes of polymeric ductile fibres which may be used
in embodiments of the present invention include any of the known
orientable polymers. In particular the oriented polymer may be an
unsubstituted or mono- or poly-halo-substituted vinyl polymer, an
unsubstituted or hydroxy-substituted polyester, a polyamide, a
polyetherketone or a polyacetal. Suitable examples include vinyl
chloride polymers, vinyl fluoride or vinylidene fluoride polymers
PHB, PEEK and homo- and copolymers of polyoxymethylene.
[0106] In the context of the present invention the high stiffness,
brittle fibre has a failure strain of less than 4%, more preferably
less than 3.5% or 3%, such as less than 2.5% or 2%. Suitable
brittle fibres include but are not limited to carbon fibre, glass
fibre, or natural fibre such as for example flax fibre, hemp,
basalt, jute, and the like. Preferably said brittle fibre is a
carbon fibre. Preferably said brittle fibre has a stiffness of at
least 30 GPa, preferably at least 40 GPa.
[0107] Preferably, the (ductile and/or brittle) fibres used in the
present invention are continuous fibres or fibres with an aspect
ratio of at least 100. In the context of embodiments of the present
invention the brittle fibre content of the hybrid composite is less
than 30 vol % or 25 vol %, preferably less than 20 vol % or 15 vol
%, more preferably less than 12 vol %, 10 vol %, or 8 vol %, such
as less than 6, 5 or 4 vol %.
[0108] In the context of embodiments of the present invention the
brittle fibre content of the hybrid composite may range between 0.5
vol % and 30 vol %, such as between 0.5 vol % and 25 vol %,
preferably ranges between 1 vol % and 20 vol %, such as between 1
vol % and 18 vol %, more preferably ranges between 1 vol % and 15
vol % or between 2 vol % and 15 vol %, even more preferably between
2 vol % and 12 vol % or between 2.5 vol % and 10 vol %, even more
preferably ranges between 3 vol % and 8 vol %.
[0109] The degree of dispersion is a measure for the mixture
intensity of fibres, e.g. two fibres, and is useful to evaluate the
dispersion of the brittle fibres in the polymeric matrix. As
indicated above, the degree of dispersion may be defined
differently for different configurations. A high degree of
dispersion can thus be obtained in different ways depending on the
hybridization configuration. In laminated composites a measure for
the degree of dispersion can be defined as the reciprocal of the
smallest repeat unit of the laminate. In intralayer configurations,
a measure for the degree of dispersion may be a reciprocal distance
between two different portions comprising such fibres and in
intrayarn a measure for the degree of dispersion may be a
reciprocal (e.g. average) distance between fibres.
[0110] Alternatively, the degree of dispersion can be evaluated by
measuring the average fibre to fibre distance using image analysis.
The closer the average (measured) (brittle) fibre to (brittle)
fibre distance is to the (theoretical) predicted fibre to fibre
separation, calculated based on the (brittle) fibre volume
fraction, the better the degree of dispersion of said fibre is
within the composite. A higher degree of dispersion means that the
constituents are more finely mixed or occur in thin layers. In
particular, a high degree of dispersion can also be obtained when
the same polymer occurs throughout the hybrid, e.g. when both the
brittle and the ductile fibres occur in all the layers that make up
the composite. By way of illustration, embodiments of the present
invention not being limited thereto, different embodiments of the
present invention will now be discussed in somewhat more
detail.
[0111] A first specific embodiment of the present provides a
fibrous self-reinforced composite material comprising (i) a
thermoplastic polymer matrix, (ii) an oriented polymeric ductile
fibre made up of said thermoplastic polymer and (iii) a brittle
fibre, wherein the brittle fibres make up less than 30 vol %,
preferably less than 25 vol %, 20 vol % or 15 vol %, of the
composite material and wherein the brittle and the ductile fibres
are organised in an interlayer configuration. Preferably, the
thickness of the layers is less than 125 .mu.m times the square
root of the ratio (230 GPa/stiffness of the brittle fibre). E.g.
for carbon fibres, the width of the layers advantageously is less
than 125 .mu.m or less. Depending on the fibres used, the width of
the layers may be less than 200 .mu.m or less than 150 .mu.m,
preferably less than 125 .mu.m, more preferably less than 100 .mu.m
or 80 .mu.m, such as less than 60 .mu.m, 50 .mu.m or 40 .mu.m. It
is understood that thinner layers result in a higher degree of
dispersion. Without being bound by theory, by using thinner layers,
the damage of the brittle fibre has less impact on the
self-reinforced layers than with thicker layers. Consequently, this
increases the residual strength after the failure of the carbon
fibre layers. The interlayer hybrid self-reinforced composite of
the present invention may be a unidirectional composite or a
multidirectional composite.
[0112] A second embodiment of the present provides a fibrous
self-reinforced composite material comprising (i) a thermoplastic
polymer matrix, (ii) an oriented polymeric ductile fibre made up of
said thermoplastic polymer and (iii) a brittle fibre, wherein the
brittle fibres make up less than 30 vol %, preferably less than 25
vol %, 20 vol % or 15 vol %, of the composite material and wherein
the brittle and the ductile fibres are organised in an intralayer
configuration. In one embodiment, the intralayer hybrid
self-reinforced composite may be a multidirectional composite,
although embodiments are not limited thereto as described above and
as shown in the examples. The brittle fibres may be oriented in one
or in more than one directions, e.g. in the weft or warp direction
or both. Preferably, the thickness of each layer is less than 125
.mu.m times the square root of the ratio (230 GPa/stiffness of the
brittle fibre). For carbon fibres, the thickness may e.g.
advantageously be less than 125 .mu.m. In some other embodiments
for other types of fibres, the thickness may be less than 200 .mu.m
or less than 150 .mu.m, preferably less than 125 .mu.m, more
preferably less than 100 .mu.m or 80 .mu.m, or even less than 60
.mu.m, 50 .mu.m or 40 .mu.m.
[0113] Optionally, an interleaved film may be present between the
layers in an inter- or intralayer configuration, e.g. to influence
the adhesion between the matrix and the brittle fibre. Such an
interleaved film may be made up of the same material as the matrix
phase or a derivative thereof, but may alternatively also be of a
different type or different polymer than the matrix phase. Further
examples of features that may occur in intralayer configurations
can also be seen in the examples discussed further down in this
description.
[0114] A third embodiment of the present invention provides a
fibrous self-reinforced composite material comprising (i) a
thermoplastic polymer matrix, (ii) an oriented polymeric ductile
fibre made up of said thermoplastic polymer and (iii) a brittle
fibre, wherein the brittle fibres make up less than 30 vol %,
preferably less than 25 vol %, 20 vol % or 15 vol % of the
composite material and wherein the brittle and the ductile fibres
are organised in an intrayarn configuration. The intrayarn hybrid
self-reinforced composite according to an embodiment of the present
invention can be a unidirectional or a multidirectional composite.
The commingled yarns can be organised as a parallel multifilament
strip or film, or as a fabric or mat, either woven, non-woven or
braided. Said fibrillated strip or fabric can further be organised
according to an inter- or intralayer configuration.
[0115] Another specific embodiment of the present invention relates
to a hybrid self-reinforced composite, comprising a brittle fibre,
preferably carbon, and a self-reinforced polyamide, wherein said
brittle fibre makes up less than 30 vol %, preferably less than 25
vol %, 20 vol % or 15 vol % of the composite material, and wherein
the fibres are organized in an interlayer, intralayer or intrayarn
configuration. Preferably, said brittle fibres are highly dispersed
within said composite material. Surprisingly, said hybrid
self-reinforced polyamide composite according to an embodiment of
the present invention showed improved flexural properties.
[0116] Another specific embodiment of the present invention relates
to a hybrid self-reinforced thermoplastic polyolefin composite,
comprising a brittle fibre, preferably carbon, and a
self-reinforced thermoplasticic polyolefin, preferably
polypropylene or polyethylene, wherein said brittle fibre makes up
less than 30 vol %, preferably less than 25 vol %, 20 vol % or 15
vol % of the composite material and wherein the fibres are
organized in an interlayer (with layer thickness less than 150
.mu.m or 100 .mu.m), intralayer or intrayarn configuration.
Preferably, said hybrid self-reinforced thermoplastic polyolefin
composite further comprises an interleaved film of said
thermoplastic polyolefin or a derivative thereof, such as a maleic
anhydride grafted PP film in a hybrid self-reinforced PP
composite.
[0117] Advantageously, the hybrid self-reinforced polyamide or
polyolefin composite according to an embodiment of the present
invention, preferably comprising a carbon fibre, combines the
properties of a self-reinforced nylon (e.g. PA6) or polyolefin
sheet (i.e. lightweight, outstanding impact performance and
strength and producable in high-volume processes) with those of
existing carbon fibre reinforced composites, which have outstanding
stiffness and strength but low toughness (especially
thermosets).
[0118] In at least some embodiments, the composite may be
characterised by one or more of the following: (i) a stiffness of
at least 6 GPa, e.g. at least 8 GPa or 10 GPa, preferably at least
12 or 13 GPa, more preferably a stiffness of at least 15 GPa;
(ii) a tensile strength of at least 250 MPa, preferably at least
300 or 350 MPa, more preferably at least 400 or 450 MPa; (iii)
impact (Izod) of at least 2000 J/m or 2500 J/m, preferably at least
3000 J/m or 3500 J/m, more preferably at least 4000 J/m (iv)
Toughness, expressed as the energy to penetration of at least 12
J/mm or 15 J/mm, more preferably at least 18 J/mm.
[0119] It is an advantage of at least some embodiments of the
present invention that the composite material may have one or more
of the following advantages:
[0120] The composite material according to an embodiment of the
present invention may have at least twice the stiffness,
substantially the same or higher strength and at least 0.8 times
the failure strain of a composite material not having the brittle
fibres but further having the same composition and
configuration.
[0121] In another aspect, the present invention relates to a method
for manufacturing a fibrous self-reinforced composite material. The
method may be advantageously be used for manufacturing a fibrous
self-reinforced composite material as described in the first
aspect, although embodiments of the present invention are not
limited thereto. The method according to embodiments of the present
invention comprises providing a thermoplastic polymer as matrix
phase, and a reinforcement phase comprising a first oriented
polymeric ductile fibre having the same type as the matrix phase
and a second high stiffness, brittle fibre. The brittle fibres
thereby make up less than 30 vol % of the composite material. Said
providing comprises, according to embodiments of the present
invention, providing said brittle fibres in a highly dispersed
within said composite material by
[0122] organising the ductile fibres and the brittle fibres in an
intralayer configuration, and/or
[0123] organising the ductile fibres and the brittle fibres in an
interlayer configuration wherein alternatingly a layer of ductile
fibres and a layer of brittle fibres is introduced and wherein the
thickness of the layer of brittle fibres is smaller than 125 .mu.m
times the square root of the ratio (230 GPa/stiffness of the
brittle fibre), and/or organising the ductile fibres and the
brittle fibres in an intrayarn configuration.
[0124] Methods according to embodiments of the present invention
thereby result in hybrid self-reinforced composites having good or
improved properties by the introduction of brittle fibres in a
highly dispersed manner. Furthermore, according to at least some
embodiments of the present invention, the properties of a hybrid
self-reinforced composite may also be good or improved by
manufacturing them and thereby adjusting the adhesion or bond
between the brittle fibres and the polymeric matrix. One embodiment
relates to a method to improve the flexural properties of a hybrid
self-reinforced composite by taking steps to increase the adhesion
between the polymeric matrix and the brittle fibre. Adhesion can be
improved by choosing a polymeric material which is more compatible
with the brittle fibre used, such as the combination
polyamide-carbon fibre. Adhesion between the layers can be improved
by adding an interleaved film of the same material as the matrix
phase or a derivative of such material, such as using a maleic
anhydride grafted PP to improve the PP-carbon fibre adhesion.
[0125] Another embodiment relates to a method to improve the
tensile properties of a hybrid self-reinforced composite by taking
steps to decrease the adhesion between the polymeric matrix and the
brittle fibre, such as by choosing a thermoplastic polymeric
material which is less compatible with the brittle fibre used (e.g.
PP and carbon fibre).
[0126] According to some embodiments of the present invention, the
method may comprise applying a hot compaction technique, such as
for example described in GB2253420 or WO1998015398, although
embodiments of the present invention are not limited to the use of
such a technique. Preferably, said method of the present invention
further may comprise the step of making an assembly of brittle and
thermoplastic polymeric ductile fibres, which may include one or
more of the following steps: a laminating step, a weaving step (to
produce a cloth), a braiding step, . . . .
[0127] For most applications of the products of this invention the
preferred processes are those that comprises a hot compaction
technique which is carried out in a manner which selectively melts
from 5 to 10% by weight of the polymer fibre material, or which
selectively melts at least 10% by weight of the polymer fibre
material, such as from 10 to 20% or 10 to 30% by weight of the
polymer or even up to 35, 40, 45 or 50% by weight. Preferably, the
hot compaction technique is carried out in a manner (process
temperature, pressure & time) allowing the generation of
sufficient matrix material by the selective melting of the
polymeric fibres to coat all the brittle fibres, while still
retaining a significant amount of molecular orientation of said
polymeric fibres. The extent of melting relating to a specific
processing temperature can easily be evaluated by e.g. Differential
Scanning calorimetry (DSC) or Wide Angle X-ray Scattering.
[0128] In a preferred embodiment the temperature at which the
fibres are compacted is a temperature between the onset of the
melting endotherm and end of the melting endotherm of a constrained
oriented polymer fibre or tape as measured by DSC. The temperature
at which the fibres are compacted is within 20.degree. C.,
15.degree. C., 10.degree. C., 8.degree. C., 6.degree. C., 5.degree.
C., 4.degree. C. or 3.degree. C. of the peak temperature of melting
of the polymeric ductile fibre i.e. the temperature of which the
endotherm measured by DSC of the polymer fibres reaches its
maximum. The minimum temperature at which the fibres should be
contacted is preferably that at which the leading edge of the
endotherm of melting of the polymeric fibre extrapolated to zero
intersects the temperature axis (onset temperature).
[0129] The pressure at which the assembly of fibres is maintained
during this stage of the process will be such as to maintain the
individual fibres in intimate contact but not such as will compact
them and in particular not inhibit the selective melting of the
polymer. In general, pressures in the range 0.5 to 5.0 MPa are
preferred.
[0130] The composite material according to the first aspect or
obtained using a method according to the second aspect may
advantageously be used for producing devices, such as for examples
structural elements, for use in one of the following applications,
embodiments of the present invention not being limited thereto:
[0131] Automotive--body panels, parcel shelves, under-shields, load
floors
[0132] Off-road vehicles--high-impact exterior panels
[0133] Marine--personal watercraft
[0134] Sports--helmets, pads, guards, shoe soles, . . .
[0135] Leisure--suitcases, loudspeaker cones
[0136] Personnel protective equipment--safety helmets,
anti-ballistic shields
[0137] Medical--orthoses, temporary supports
[0138] Construction--shuttering, formwork
[0139] By way of further illustration, embodiments of the present
invention not being limited thereto, a number of examples of
composite materials are discussed and experimental results thereon
are described, these experimental tests illustrating features and
advantages of embodiments of the present invention.
Example 1
Multidirectional Hybrid Self-Reinforced CF/PP Composites
(Interlayer)
[0140] Multidirectional hybrid SRPP were produced using weaves of
PP tapes. Hence, the hybrids also have fibres transverse to the
loading direction. Textreme weaves (Oxeon) were impregnated to form
Textreme prepregs (layer thickness of ca. 100 .mu.m) and various
lay-ups were made and tested. These layups are indicated by their
stacking sequence, where "P" stands for SRPP and "T" for the
Textreme prepregs. The tensile curves are shown in FIG. 7. Because
of the ductile behaviour after the carbon fibre failure, the area
underneath the tensile curve is increased compared to the
non-hybrid lay-up TTTT. This means the energy absorbed until
tensile failure is increased. This is an indication for an improved
impact resistance.
[0141] Moreover, the failure strain of the carbon fibres seems to
increase. When compared to the TTTT lay-up (ca. 1.5%), the PTP and
PPTPP show an increase in failure strain of 18% (ca. 1.8%) and 12%
(ca. 1.7%), respectively.
[0142] The effect of interleaved films was also investigated for
interlayer CFPP/SRPP hybrids. The PTTP lay-up has a low interlayer
adhesion. This interlayer adhesion can be increased by adding films
in between. This was done by adding PP and MAPP films in between
the layers. The tensile results are shown in FIG. 8. The addition
of film results in a lower and smaller second part of the curve.
The increased interlayer adhesion transfers a bigger part of the
damage of the CFPP to the SRPP. Moreover, the delamination has more
difficulties in developing over the sample, which results in a
localization of the damage. The effect of the interleaved films
seems to be more severe for MAPP films, which have an even better
adhesion.
[0143] Flexural tests were also performed on some layups, i.e. a
thin (PTTP) and a thick layup (PTPTPTPTPTPTP), with and without
interleaved films. Addition of the films increases the flexural
strength of PTTP from 93 MPa (no film) to ca. 130 MPa (for PP film)
or 135 MPa (for MAPP film). The thick layup also shows an, albeit
smaller, increase: from ca. 120 MPa (no film) to ca. 140 MPa (for
PP film). All these samples failed by delamination of layers and
buckling underneath the loading nose. An increased interlayer
adhesion by adding films makes the development of these
delaminations more difficult. This results in an increased flexural
strength. Even with films, the delaminations are responsible for a
low flexural strength compared to the tensile strength.
Example 2
Hybrid Self-Reinforced Polyamide Composites (Inter &
Intralayer)
[0144] Study outline. Novel self-reinforced carbon fibre hybrid
composites were developed based on Nylon 6 (PA) in a intra-layer
and interlayer setup. In the intra-layer hybridisation, highly
stretched PA6 tapes are co-woven with PA6/T700 carbon fibre prepreg
tapes, to produce a cloth, which is then processed using the hot
compaction technique. In the inter-layer technique, the T700/PA6
prepreg tapes were simply laminated together with sheets of
self-reinforced PA6. In the study, the volume fraction of the
carbon fibres, compaction conditions (particularly compaction
temperature) and the weave style were varied.
Composite Preparation:
[0145] Continuous carbon fibre (type T700) reinforced PA prepreg
tapes were obtained from Toray Europe with a carbon fibre volume
fraction of .about.50%. The tapes were 3.2 mm wide and 0.28 mm
thick. Oriented PA6 tapes were produced at Leeds using a purpose
built draw frame. Experiments showed that the best properties for
the drawn tape were from by a two stage drawing procedure. For the
first draw, the ratio was 7:1 at a temperature of 120.degree. C.,
followed by a subsequent draw of 2:1 at 140.degree. C.
Intra-Layer Configuration:
[0146] The two tapes (highly drawn PA6 and T700 carbon/PA6 prepreg)
were woven on a hand loom to produce cloth. For the pure PA6, both
the warp and weft were from the PA6 drawn tapes. For the hybrid
co-woven cloth, the PA6 drawn tapes formed the warp and the
T700/PA6 tapes the weft. The cloths produced were around 200 mm
wide and 800 mm long. Next, the cloths were made into hybrid
samples using the hot compaction process. The optimum compaction
temperature was found to be 202.+-.1.degree. C. For these tests,
the carbon fibre tapes were arranged in one direction (the
subsequent testing direction). By using different amounts of the
two cloths, a range of carbon fibre volume fractions were obtained
(from 0 to 30%).
Inter-Layer Configuration:
[0147] For the interlayer samples, lengths of the PA6/T700 prepeg
tapes were laminated on the outside of a pure PA6 hot compacted
sheet.
Tensile Tests--Intralayer Configuration:
[0148] As described above, in the first series of tensile tests
(performed according to ASTM standard D3039) the composites were
made with the carbon fibres all laid in the same (testing)
direction. They were then interleaved with layers of pure PA6 woven
cloth to change the fibre volume fraction. These latter samples can
be considered to be a mixture of intra and inter-layer
hybridisation. FIG. 2 shows the results of tensile tests and the
various lay-ups in curves 202, 204, 206 and 208. All the samples
tested, even at the lowest carbon fibre volume fraction of 4%,
showed brittle failure (strain to failure of ca. 2%), that is
catastrophic failure of the sample once the carbon fibres break.
Only at low carbon fibre content, we see an increase in the failure
strain. This is very different behaviour to a 100% PA6 SRP
composite sheet, which shows a much lower modulus but a strain to
failure of 10%.
[0149] In the next set of experiments, the carbon fibre prepreg
tape was cut in half and then co-woven with the oriented PA6 tapes.
This had the effect of reducing the amount of carbon fibre at any
location by 50%. FIG. 3 shows that for the samples tested in
tension with this configuration, some load bearing capability
remained after the carbon fibres were broken. This suggests that
separating the carbon fibres could mediate the shock effect of the
carbon fibres breaking and releasing their stored energy.
Interestingly, when the carbon fibre tapes are split in two, a more
ductile behavior is observed. After the carbon fibre layer breaks,
the SRPA6 layers still continues to carry some load, although it's
limited. This clearly shows the importance of intensily mixing both
fibre types. In FIG. 3, curve 302 illustrates the reference
situation, whereas curve 304 illustrates the situation wherein the
carbon fibre prepreg tapes were cut in half.
Tensile Tests--Interlayer Configuration:
[0150] For the first interlayer tests, samples were made by placing
the prepreg tape on the outside of the SRPA6 woven layers and then
using the hot compaction technique as above. These samples all
showed brittle failure similar to that described in the previous
section for the Intra-layer samples. A second set of tests were
carried out, but this time laminating the CF prepreg tapes to the
outside of the SRPA6 sheet using a cyanoacrylate adhesive (FIG.
4).
[0151] The results in FIG. 4 show that for this simple laminate,
the carbon fibres can break without damaging the SRP PA6 sheet,
which then proceeds to carry load until over 10% failure strain. In
this configuration, the two components effectively behave in
parallel (more or less independently from each other), with the
resulting hybrid combining the best aspects of the two component: a
stiffness of 11 GPa, a strength of 230 MPa and a failure strain of
11%. This kind of stress-strain curve shows that the material has a
high stiffness and strength in regular use and can still absorb a
lot of energy when impacted.
Bending Tests--Intra Layer Configuration:
[0152] Bending tests were also carried out on the intra-layer
(co-woven) samples under ASTM D790. Interestingly, the co-woven
nylon hybrids were found to be ductile in flexure, compared to
their brittle behaviour in tension. It was seen that the sheets
retained their load carrying capacity even after the carbon fibre
broke. FIG. 5 shows a typical comparison of a tension test and a
flexure test on the same hybrid intra-layer sample. The flexural
modulus was measured to be lower in flexure (as the carbon is
located towards the centre of the sample), but remained intact once
the carbon layer had fractured.
[0153] FIG. 6a and FIG. 6b summarises the tensile respectively
bending properties of the intra-layer hybrids compared to the pure
SRPA6 sheet. In both cases, the hybrid sample has a much improved
stiffness and strength. Whereas in tension, the sample breaks once
the carbon fibres break, in bending this does not occur, and the
sample is not seriously damaged when the carbon fibre fraction
breaks.
[0154] For the intra-layer (co-woven) hybridisation strategy,
samples were found to be brittle in tension, that is once the
carbon fibres broke, the samples as a whole broke: the carbon fibre
failure presumably dramatically damages the SRPA6, which is unable
to deal with this damage. This behaviour can be improved by a smart
placement of the carbon fibre layers; limiting the amount of carbon
fibre and/or decreasing the adhesion of the carbon fibre, such as
e.g. by using PP instead of PA (see Example 4). However, in
bending, this was not the case, with the hybrid samples remaining
load bearing up to a high failure strain even after the breakage of
the carbon fibres. For the interlayer hybridisation, an alternative
strategy was adopted, where the prepreg tapes were located on the
exterior of the SRPA6 sheets. In tensile testing this combination
was found to behave in parallel, so that the composites showed
ductile behaviour even after the breakage of the carbon fibres.
Example 3
Hybrid Self-Reinforced Polypropylene Composites (Intralayer)
Comparison Between Cowoven Hybrid SRPA6 & Hybrid SRPP:
[0155] Using a similar setup as in example 2, a cowoven hybrid
CF/SRPP composite was made (carbon fraction ca. 4%) & the
result is depicted in FIG. 9. Although the mechanical properties at
low strains are comparable, the PP hybrid composites show a ductile
behaviour, while the nylon sample show a more brittle & less
ductile behaviour. It seems the failure of the carbon fibres
damages the SRPP to a lesser extent.
Tensile Properties of Cowoven Hybrid SRPPs with Varying CF
Content:
[0156] Hybrid weaves of PP tapes and CFPP tapes were prepared as
follows. First, a UD cloth was created, in which carbon fibre is
only in 1 direction, while the PP tapes are in both directions. The
carbon fibre volume fraction was diluted by adding additional SRPP
weaves on the outside. Next, these configurations were hot
compacted.
[0157] FIG. 10 shows the results of tensile tests. The following
clear trends appear:
[0158] The failure strain of the carbon fibre layers increases with
lower fibre volume fractions. Surprisingly, this failure strain
enhancement is about 100%.
[0159] The SRPP layers are almost unaffected by the fibre volume
fraction of carbon fibre. The plateau and the failure strain are
almost the same as for the non-hybrid SRPP.
[0160] In a second setup, UD cloths were woven at a carbon fibre
volume fraction of about 14%. Eight of these cloths were stacked in
0.degree. and 90.degree., to obtain a balanced and symmetric
laminate. Carbon fibre is hence present in 2 directions. This was
done twice: once with and once without an interleaved PP film. The
results are shown in FIG. 11.
[0161] Without the interleaved PP film, the two constituents of the
hybrids behave predominantly independent of each other. At low
strain, the behaviour is dominated by the CFPP. When the CFPP
breaks, the SRPP starts carrying the main load. The CFPP still
contributes a little bit of stress, since the SRPP cannot reach 50
MPa at a strain of 2%. The tensile behaviour changes when an
interleaved film is added. The stress at both low and high strains
increases. Presumably because of the improved adhesion, the damage
stays localized. While the delamination in the hybrids without film
can spread over the entire sample, this is not the case for the
hybrids with film. The improved adhesion also results in a larger
contribution of the carbon fibres, even after 2% strain. This
results in a higher stress at higher strains.
[0162] Some hybrid CF/SRPP's were also tested in three point
bending mode. In contrast to what was found for the hybrid CF/SRPA6
composites (FIG. 6), the flexural strength of the hybrid SRPP
composite is low compared to the tensile strength. Without being
bound by theory, the low flexural properties may be due to the low
internal adhesion of the hybrid SRPP. This can be improved by
adding an interleaved PP film. However, the low adhesion seems to
be good for the tensile and probably also for its impact
properties. These results clearly suggest that controlling the
adhesion (such as by changing the material and/or inclusion of an
interleaved film) leads to different composite materials with a
different range of tensile and flexural properties.
Example 4
Hybrid Self-Reinforced Polyamide Composites (Intrayarn)
[0163] CF/PA12 commingled yarns (with CF content of 25 vol %) were
obtained from Schappe Techniques (France). Analysis of the
microstructure of the yarns showed that the fibres are not
continuous. However, the shortest carbon fibre still measure a few
centimeters, resulting in an aspect ratio well over 100. Therefore,
no significant influence on stiffness or strength is expected.
[0164] Hot-compacting the commingled yarn at 190.degree. C. ("UD
190.degree. C.") results in the complete melting of the PA12
fibres. This way, a well-impregnated non-hybrid thermoplastic
composite is obtained (with CF fibres in a PA12 matrix).
[0165] Hot compacting the commingled yarn in the range
173-176.degree. C. results in the selective melting of the outside
of the PA12 fibres, while the inner core of the PA12 fibres
maintains its good mechanical properties. This way, a
self-reinforced CF/SRPA12 composite is obtained. FIG. 12 shows the
tensile behaviour of the CF/PA12 intrayarn hybrids. The
hot-compaction temperature does not seem to affect the tensile
behaviour to a large extent, although the strength and failure
strain seem to be slightly lower for samples compacted at
173-176.degree. C.
[0166] The flexural behaviour of these intrayarn hybrids are
however significantly different from the fully molten "UD
190.degree. C." sample. The "UD 190.degree. C." sample behaves
completely brittle (with failure of the composite at a strain of
ca. 2%). The sample compacted at 173.degree. C. on the other hand
still has a ductile part of the first load drop. By choosing the
right compaction conditions, carbon fibre composite can be obtained
to behave ductile in bending.
[0167] In a further set of experiments, the intrayarn filaments
were braided into a cloth. Samples were hot compacted using the
temperature range suggested by the above first set of tests.
[0168] The results are presented in FIG. 13 and showed that while
at a low compaction temperature the sample showed brittle failure,
above a critical temperature (.about.175.degree. C.) the sample
remained load bearing once the carbon fibres had broken. The
samples made at the higher temperature also showed a significantly
increased area under the tensile stress-strain curve, indicating
improved toughness over the other hybrid nylon based samples.
[0169] FIG. 14 shows the tensile properties of intrayarn samples
made with three different weave styles: a 0/90 laminate (with
completely straight fibres), a hand woven plain weave sample (with
significant crimp) and braided cloth (4.times.4 twill). The results
show that the weave style has a significant effect on the tensile
properties. The 0/90 cross ply showed the highest stiffness and
strength but a low failure strain. The hand woven cloth showed both
a low modulus and strength and still a low failure strain. The
braided cloth showed a lower modulus and strength but a much higher
failure strain, both at the point where the carbon fibres break
(.about.2%) and then at final fracture (.about.8%).
Example 5
Hybrid Self-Reinforced CFPP/SRPP Hybrids
[0170] The influence of the dispersion was studied in CFPP/SRPP
hybrids. In this example the influence of the degree of dispersion
was studied by attaching continuous CFPP prepregs to SRPP weaves
using heat resistant tape and by altering their distribution. Using
this setup, the degree of dispersion could be easily varied by
splitting the CFPP bands and by changing the spacing between the
tapes at the same time so that the same global fibre volume
fraction was maintained, thus studying the effect of dispersion and
not of amount of fibres. In the produced samples, this was 18% in
both cases.
[0171] The tensile diagrams are shown in FIG. 15. No significant
difference was found for the first peak, which is related to the
CFPP peak. Some differences can be observed in the second part of
the tensile diagram. In the samples with 3 mm wide bands, the
stress falls back to 58.+-.2 MPa, while this is increased to
70.+-.1 MPa if the bands are only 1.5 mm wide. The stress level in
the second part is hence higher if the prepregs bands are narrower.
This means that a higher degree of dispersion is able to spread out
the CFPP failure more, which results in a lower damage to the SRPP.
Moreover, the CFPP is able to contribute to some stress transfer in
the second part of the diagram. If the dispersion is increased,
this contribution is increased. This demonstrates the relevance of
the degree of dispersion for maintaining the SRPP ductility in
SRPP/CFPP hybrid composites. It is expected that at even better
dispersions, this effect will be stronger.
[0172] The influence of the carbon fibre volume fraction in an
intralayer SRPP/CFPP hybrid was also studied. Three different
intralayer SRPP/CFPP weaves were produced at different carbon fibre
volume fractions: 3%, 8% and 15%. The tensile diagrams for these
weaves were compared with the SRPP cloth with 0% carbon fibre, as
shown in FIG. 16. When using a high carbon fibre volume fraction,
the CFPP adds more matrix, which caused a stronger interlayer
bonding. This drastically reduces the failure strain, in the case
of the 15% cloth. This is further shown in FIG. 17.
[0173] The penetration impact resistance of the same materials is
shown in FIG. 18 illustrating that the toughness of the SRPP can be
maintained, but only if the carbon fibre volume fraction is
sufficiently low.
Example 6
Carbon Fibre Composites Hybridised with Self-Reinforced
Polypropylene
[0174] Example 6 illustrates a further example wherein a good
balance between toughness on the one hand and stiffness and
strength on the other hand is obtained. The example illustrates
hybridisation of carbon fibre-reinforced composites with ductile
fibres. The following example illustrates particular features,
embodiments of the present invention not being limited thereto.
[0175] The applicability of SRPP was previously hampered by the
limited stiffness and strength, but this can be improved by
hybridisation, which can be done in the three different
configuration as described above. Results will be presented for
intralayer hybrids of continuous carbon fibre polypropylene (CFPP)
with SRPP. The highly oriented PP tapes in SRPP add a tough
material to the brittle CFPP. Polypropylene (PP) tapes having a
draw ratio between 1:10 and 1:15, a stiffness of 8-10 GPa, and a
strength of 500-600 MPa. The unidirectional carbon fibre
polypropylene prepregs were 300 .mu.m thick and 55 mm wide, but
were slit manually to 2.5 mm width. The fibre volume fraction is 47
vol % and the type of fibres is T700S. A 20 .mu.m thick PP film was
also provided. This film has a melting point of 163.degree. C. and
consists of the same PP grade as the tapes.
[0176] The PP tapes and CFPP prepreg tapes were woven into hybrid
fabrics. Folding of the tapes was avoided by using a hand loom. The
warp direction was composed of only PP tapes, while 1 out of 4
tapes in the weft direction was a CFPP prepreg. This fraction of
CFPP prepregs to PP tapes was chosen to give a carbon fibre
fraction in the final composite sheet of around 20%.
[0177] Two weave patterns were woven to assess the influence of the
weave pattern and crimp. To obtain a large difference between both
patterns, a plain weave was compared to sateen 8/3 weave. The plain
weave pattern has the highest possible crimp of all standard
patterns. The sateen 8/3 pattern has a low amount of cross-overs
and hence a low crimp. After hot compaction, each layer will have
an average thickness of around 150 .mu.m. A plain weave without
carbon fibre prepregs was also woven. This will be used as
reference material to compare the hybrid composites with.
[0178] A total of 8 layers of the fabric were stacked on top of
each other in a 0.sub.8 or (0-90-0-90).sub.s layup. The weft
direction is labelled as the 0.degree. direction, as this is the
stiffest and strongest direction. The layups are abbreviated to
0.degree. and 0.degree./90.degree. respectively. Note that the
layup for the reference SRPP fabric is irrelevant, since the
0.degree. and 90.degree. are identical.
[0179] The layup is put in between two 1 mm thick copper cover
plates and inserted into a preheated press at 188.degree. C. It is
hot compacted for 5 minutes at 45 bar pressure, after which it is
cooled down to 40.degree. C. in 4 minutes.
[0180] The reference CFPP material was pressed in a copper channel
mold to avoid flow at the edges. The processing conditions are the
same, apart from a lower pressure. Pressure was lowered to 5 bar to
prevent the material from flowing out of the mold, which would
misalign the fibres. The higher pressure for the SRPP reference and
hybrids is needed to overcome the entropic shrinkage of the PP
tapes during hot compaction.
[0181] In one layup, seven interleaved PP films were inserted in
between the eight hybrid fabrics. Apart from decreasing the carbon
fibre volume fraction from 22% to 19%, the films also create more
matrix material. This increases the adhesion between the layers and
widens the temperature window for hot compaction.
Tensile Tests
[0182] Quasi-static tensile tests were performed according to ASTM
D3039. Tensile samples of 250.times.25 mm were waterjet cut to
minimise the damage to the sample edges. The strain was measured by
averaging the surface strain using digital image correlation. After
the carbon fibre failure, the surface is damaged and the surface
strain cannot be measured anymore. To solve this problem, the
crosshead displacement is used to calculate the strain after the
carbon fibre failure. This correction is accurate due to two
reasons. Firstly, the error in this correction is proportional to
the load, which shows only a small variation. Secondly, the
correction was verified on samples without damaged surfaces.
[0183] The tensile modulus is calculated as the slope between 0.1%
and 0.3% strain. The strength is calculated at two different
strains: the strain at which CFPP fails and the strain at which
SRPP fails. Both these strengths and the corresponding strains are
labelled as I and II. These strains and strengths are illustrated
in FIG. 19.
[0184] Matrix burn off tests were performed according to ASTM
D2584. The samples are heated in porcelain crucible until the PP
matrix ignites. The samples are then inserted into a muffle furnace
for eight hours at 450.degree. C. The fibre weight fraction is
calculated based on the sample weight before and after burn off.
This is converted into a fibre volume fraction by assuming a
density of 1800 kg/m.sup.3 and 920 kg/m.sup.3 for CF and PP,
respectively.
[0185] The described hybrid composites consist of two components:
CFPP and SRPP. FIG. 20 illustrates the tensile diagrams of both
reference materials. CFPP demonstrated a high stiffness and
strength, but a low failure strain. This is in strong contrast with
the low stiffness and strength of SRPP. These lower tensile
properties are compensated by the increased failure strain.
[0186] Since the weaves only have carbon fibre in the 0.degree.
direction, the layup is vital for the mechanical properties of the
hot compacted sheets. Therefore, 0.degree. and 0.degree./90.degree.
layups were hot compacted and tested. Their tensile diagrams are
presented in FIG. 21, while the second and third columns of table 2
summarize the tensile properties.
[0187] By way of illustration, the tensile properties of the
reference materials are shown in the table below.
TABLE-US-00001 CFPP SRPP Stiffness (GPa) 91 .+-. 5 3.0 .+-. 0.2
Strain (%) 1.6 .+-. 0.1 14.3 .+-. 1.7 Strength (MPa) 1227 .+-. 70
117 .+-. 5
[0188] Both layups demonstrated a distinct CFPP peak at about 1.5%
strain, followed by a SRPP tail. The properties in the second part
of the stress-strain diagram are hardly affected by the layup. The
SRPP seems to remain unaffected by the energy released upon CFPP
failure. Both components can be considered as acting independently.
This is possible because of a combination of the high SRPP
ductility and the well-known low adhesion between CF and PP.
[0189] A crucial difference between both layups is the amount of
carbon fibre in the tensile direction. The 0.degree./90.degree.
layup only has half of the carbon fibres in the tensile direction
compared to the 0.degree. layup. This results in a stiffness
difference of a factor two. Based on the carbon fibre volume
fraction of 22% and a CF stiffness of 230 GPa, a stiffness of at
least 50.6 GPa would be expected for the 0.degree. layup. The
measured 33.5 GPa significantly differs from the expected value,
however. This difference has two reasons. Firstly, the modulus of
CF is typically measured between 0.5 and 0.7% strain, while the
composite modulus is measured between 0.1 and 0.3% strain. The CF
modulus increases by about 20% between 0% and 0.7%, which means the
expected modulus may be reduced to 40.5 GPa. Secondly, the PP tapes
have a high tendency to shrink during processing. This can induce
misalignment of the carbon fibres, further reducing the composite
tensile modulus. Finally it is interesting to note that the SRPP
portion of the tensile stress-strain curve is independent of the
lay-up, supporting the explanation that the two components are
acting independently.
[0190] The measured data can also be compared to the predicted
behaviour for the 0.degree. layup. The tensile diagram of this
layup is easier to predict, as it has no carbon fibres in the
transverse direction. The prediction is a rule-of-mixtures based on
the experimental reference material data (see FIG. 20), weighed by
their relative volume. This assumes both components behave in
parallel and do not interact with each other. CFPP's volume can be
estimated by dividing the carbon fibre content of the hybrid
composite (22%) by the fibre volume fraction of the CFPP prepregs
(47%). This results in a relative ratio of 47% CFPP and 53% SRPP.
These ratios are used as weighing factors for the stress-strain
diagrams of the reference materials.
[0191] FIG. 22 compares the predicted results with the
measurements. The CFPP peak is accurately predicted, both for
stiffness and strength. However, a large difference is observed
after the CFPP failure. The prediction assumes that CFPP stops
carrying load, which means the stress falls back to the level of
SRPP at that strain. This results in a vertical stress drop to
about 15 MPa. The measured stress is, however, higher, which means
the carbon fibres are still carrying a part of the load after they
are broken. This results in positive deviation from the
rule-of-mixtures, which is often referred to as a positive hybrid
effect. This positive effect decreases with increasing strain and
disappears at about 12% strain.
[0192] A second discrepancy between prediction and measurement is
observed around 20% strain. The prediction yields a higher stress
level, which means that some damage to SRPP has occurred. In
conclusion, the layup affects the mechanical properties of the
hybrid mainly through the orientation of CFPP. The SRPP part of the
tensile diagram remains largely unaffected by the layup.
[0193] A plain weave is compared to a satin weave to assess the
influence of the amount of crimp. The plain weave has more
cross-overs, resulting in higher out-of-plane orientation of both
the carbon fibres and the PP tapes. FIG. 23 depicts the tensile
diagrams of both weave patterns for 0.degree./90.degree. layups,
similar to FIG. 11.
[0194] No significant differences were found in the stiffness and
strength, which means that the crimp is not affecting the behaviour
of the carbon fibres. This can be understood from the dimensions of
the CF prepregs and PP tapes. The width over thickness ratio is 8
and 50 respectively, resulting in a low crimp for both weave
patterns.
[0195] Small differences can be observed in the second part of the
tensile diagram. The satin weave has a lower strain II and strength
II. This part of the tensile diagram is determined by the damage in
SRPP. The cross-overs in the weave pattern act as crack stoppers
and tend to limit the extent of the CFPP damage. Since the plain
weave has more cross-overs, the CFPP failure damages SRPP over a
smaller region. This results in higher strain II and strength II
for the plain weave. Moreover, the higher number of cross-overs in
the plain weave also results in a wavier surface than in the satin
weave. The plain weave's wavier surface results in a higher
resistance against delamination and a delayed onset of damage.
[0196] The table below illustrates the tensile properties of the
hybrid composites.
TABLE-US-00002 Layup 0.degree. 0.degree./90.degree.
0.degree./90.degree. 0.degree./90.degree. Pattern Plain Plain Satin
Plain Film No No No Yes Stiffness (GPa) 33.5 .+-. 3.0 16.1 .+-.1.7
16.1 .+-. 2.0 17.8 .+-. 0.6 Strain I (%) 1.5 .+-. 0.1 1.4 .+-. 0.1
.sup. 1.5 .+-.0.2 1.6 .+-. 0.1 Strength I (MPa) 522 .+-. 28 245
.+-. 20 252 .+-. 11 280 .+-. 24 Strain II (%) 13.9 .+-. 0.2 12.1
.+-. 0.2 10.7 .+-. 0.6 6.3 .+-. 3.3 Strength II (MPa) 69.2 .+-. 4.4
84 .+-. 2 75 .+-. 2 92 .+-. 9
[0197] In conclusion, a plain weave pattern results in more ductile
behaviour, while the stiffness and strength remain unaffected.
[0198] Interleaved films were inserted between the hybrid fabrics
to increase the amount of matrix created during the hot compaction.
This increases the interlayer bonding and hence the resistance
against delamination. The tensile diagrams are shown in FIG.
24.
[0199] The interleaved films slightly improve the strain I and
strength I and result in a sharper CFPP peak. The additional matrix
improves the compaction quality and improves the bonding. The
latter can delay the onset of failure and increases the sharpness
of the peak.
[0200] The largest difference is observed in the SRPP part of the
tensile diagram. The interleaved film increases the strength II,
but dramatically decreases the strain II. This can be understood
from the difference in adhesion, which determines the extent of the
damaged region. In the composite without the films, the adhesion is
low, and the damage spreads out over the entire length of the
sample. This prevents the strain from localising in a small region
and allows SRPP to be strained independently from CFPP. In
composites with interleaved films, however, the improved adhesion
limits the extent of damaged area, which localises the applied
strain over a smaller length. Locally, the ultimate failure strain
is reached before the global ultimate failure strain is reached. At
the same time, the improved adhesion allows some of the carbon
fibres to contribute to the stress even after the first peak. This
results in the increased strength II.
[0201] In conclusion, the strength of CFPP and the SRPP peak is
increased by interleaved films. The failure strain of SRPP is,
however, dramatically decreased.
* * * * *
References